A wind turbine includes a rotor having multiple blades to transform wind energy into rotational torque that drives a generator, which is coupled to the rotor through a drive train and gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the mechanical energy to electrical energy, which is fed to a utility grid.
The amount of power that may be produced by a wind turbine is typically limited by structural limitations (i.e. design loads) of the individual wind turbine components. Further, the loads experienced by the wind turbine may depend on a number of factors, including wind speed, wind peaks, wind turbulence, wind shear, changes in wind direction, density in the air, and similar. Referring to FIG. 1, modern utility-grade wind turbines are generally operated in accordance with a design power curve wherein in a first region (Region I), the wind speed is too low to warrant turbine operation and the turbine blades are pitched to a full feather position corresponding to the pitch angle that produces minimum aerodynamic torque. At a wind speed sufficient for start-up (Vcut-in), the blades are pitched to a Region II nominal pitch angle wherein a maximum aerodynamic lift is generated to produce torque and turn the rotor. In Region II, the wind speed and generator torque are below “rated”, and blade pitch is generally held constant at an optimal angle to produce maximum aerodynamic power. With an increase in wind speed in Region II, power captured by the wind turbine increases along with mechanical loads on the turbine structure and components as illustrated in FIG. 2.
At “rated” wind speed (Vrated), the wind turbine reaches its rated power (Prated) in Region III of the design power curve. In this region, the wind turbine power is limited to rated power to maintain the machine loads within design limits. Generator torque is held constant and blade pitch is controlled to regulate turbine speed at Vrated.
In turbulent wind conditions, wind turbines may experience mechanical loads higher than design loads. As such, conventional control strategies either shut down the wind turbine completely or implement a standard “de-rate” of rated power in an effort to maintain operation of the wind turbine without exceeding design loads. The term “de-rate” as used herein is understood to mean producing less power than the rated power during full load operation. For example, as shown in FIG. 1, curve 104 represents the design power curve, whereas curve 106 represents the de-rated power curve. Similarly, as shown in FIG. 2, curves 204 and 206 represent the corresponding loading curve and de-rated loading curve, respectively. As shown, the loading curve 204 increases from Vcut-in to a maximum load 202 until the wind turbine reaches rated power, at which point the loading curve 204 begins to decrease. De-rated loading curve 206 follows a similar pattern, however, the maximum load 216 and all other loads represented by loading curve 206 are decreased by de-rating the rated power such that design loads are not exceeded. The dotted lines 208, 210, 212, 214 correspond to a +/− standard deviation of the loads acting on the wind turbine.
In other control strategies, the wind turbine power may also be “up-rated” when normal operation produces loads much lower than design loads. The term “up-rate” is understood to mean producing more than nominal power during full operation. Up-rating a wind turbine is advantageous under benign environmental conditions, when wind conditions are smooth, such that power output may be increased without overloading wind turbine components. It should be understood that “wind turbine component” is meant to encompass any portion of a wind turbine, including, but not limited to a rotor blade, a rotor, a hub, a pitch bearing, a yaw bearing, a bed plate, a main frame, a generator frame, a nacelle, a main shaft, a generator, a gearbox, or a tower of the wind turbine.
The previous control strategies described herein are effective at reducing loads acting on the wind turbine; however, they still involve certain disadvantages. For example, providing a flat de-rate or up-rate (with respect to wind speed) based on monitored loading conditions results in a loss of potential power production as indicated by gap 105 (FIG. 1). More specifically, an extreme gust or high turbulence condition may be detected over a short period of time, causing the control system to de-rate the wind turbine regardless of whether the wind conditions subsequently improve. As a result, current control strategies lose potential power output by not adjusting the initial de-rate in response to improved conditions or operation region.
Accordingly, an improved system and method for controlling a wind turbine would be welcomed in the technology. More specifically, a system and method for controlling a wind turbine that involves optimizing the power output while also maintaining loadings of individual wind turbine components below design loads would be advantageous.